A police spokesman said the child is in serious condition at a hospital in London.
Rafid Jihad, 28, of Windsor was taken into custody and charged with attempted murder shortly after the incident.
Police blocked off a section of Church and Erie streets, where the incident happened, late Friday morning.
Sgt. Matt D'Asti of the Windsor Police Service said Friday the boy was walking by the suspect's house when the alleged attack happened.He said the boy lives down the street from where the attack took place.
D'Asti said the boy was struck in the head several times with a hammer. Responding officers found the suspect sitting by the side of the road and he was arrested without incident, he said.
"This easily could have been, you know, much worse. We still don't know the exact extent of the injuries to this young boy. The latest we've heard is that his condition is serious. We know that he has multiple skull fractures and we know that his injuries are serious enough to have him transported to London," D'Asti said at the time.
D'Asti said the boy was off school for a PA Day, but that police were not aware of any conversation that may have taken place between the victim and the suspect, or whether the two know each other.
The attack is being investigated by major crimes detectives. D'Asti said police will be talking to neighbours to determine a possible motive.
At least one neighbour said he noticed the suspect was acting strangely in recent days.
"He liked to pace up and down the street for hours at a time, with a hoodie on, the hood pulled up," the man told CBC News. "He starting doing that in the last couple of weeks. It's kind of a little unnerving, actually."